Short Circuit and Overload Protection Devices …

1 Short Circuit and Overload Protection DevicesWithin an Electrical SystemInformation Sheet # 07 Your Reliable Guide for Power SolutionsThe installation information provided in this information sheet is informational in nature only, and should not be considered the advice of a properly licensed and qualified electrician or used in place of a detailed review of the applicable National Electric Codes and local codes. Specific questions about how this information may affect any particular situation should be addressed to a licensed and qualified electrician.

2 To fulfill our commitment to be the leading supplier and preferred service provider in the Power Generation Industry, the Central Power Systems & Services team maintains up-to-date technology and information standards on Power Industry changes, regulations and trends. As a service, our Information Sheets are circulated on a regular basis, to existing and potential Power Customers to maintain awareness of changes and developments in engineering standards, electrical codes, and technology impacting the Power Generation IntroductionThe designer of an electrical system has the responsibility to meet code requirements and to ensure that the equipment and conductors within a system are protected against current flows that will produce destructive temperatures above specified rating and design limits.

3 This information sheet discusses protective Devices that are used within a system, how they work and where they are Overcurrent Protection Devices : Protection against temperature is termed overcurrent Protection . Overcurrents are caused by equipment overloads, by Short circuits or by ground faults. An Overload occurs when equipment is subjected to current above its rated capacity and excessive heat is produced. A Short Circuit occurs when there is a direct but unintended connection between line-to-line or line-to-neutral conductors.

4 Short circuits can generate temperatures thousands of degrees above designated ratings. A ground fault occurs when electrical current flows from a conductor to uninsulated metal that is not designed to conduct electricity. These uninsulated currents can be designer has many overcurrent Protection Devices to choose from. The two most common are fuses and Circuit breakers. Many Circuit breakers are also known as molded case breakers or : A fuse is the simplest form of overcurrent protective device but it can be used only once before it must be replaced.

5 A fuse consists of a conducting element enclosed in a glass, ceramic or other non-conductive tube and connected by ferrules at each end of the tube. (See Diagram 2) The ferrules fit into slots at each end to complete a split in a Circuit . Excess current flowing through the fuse melts the device s conducting element and interrupts current are rated by the amperage they can carry before heat melts the element. The fuse is ideal for Protection against Short circuits. Short circuits produce enough amperage to vaporize a fuse element and break connection in one cycle of a 60-cycle system.

6 Fuses are more commonly used in Devices connected to a system than within the system s Breakers: Now, conductors in systems are usually protected by Circuit breakers. Tripped Circuit breakers can be reset after the fault is cleared, an advantage over fuses that must be replaced. (Continued over) Tripper BarTrip MechanismManual Trip ButtonTrip Mechanism LockedContacts ClosedElectromagnetShort CircuitSensing ElementABIn the ON position, as above, the trip mechanism keeps the Circuit closed and lets current flow from A to B.

7 The trip mechanism can be engaged manually foroverload by thermal sensing and Short Circuit by an electromagnetic device. There is also a manual trip Energized Circuit PositionArmatureElectromagnetShort Circuit Protection is provided by the electromagnet produces a magnetic field sufficient topull the armature only when Overload amperages arereached. Tripping occurs when the armature strikes the tripbar. This cuts current flow and releases the Magnetic Short CircuitThermal OvercurrentSensing ElementCurrent flowing through the bimetal strip causes it to heat up.

8 When a certain heat is reached, the strip bends andoperates the trip mechanism. The strip is calibrated to startbending when Overload amperage is reached. The higher the current flow, the quicker the bimetal trips the Thermal Overload PositionBimetal StripDiagram 1 - Thermal and Magnetic Trip Elements of a Circuit BreakerCopyright 2006 PLC Enterprises, LLC (Continued from previous page) Molded Case Circuit BreakerA molded case Circuit breaker, or MCB, has two distinct operating components. (See Diagram 1)Thermal Trip Component: This component is a bimetal strip that carries the current.

9 When the current exceeds a predetermined limit, heat produced by the excess current causes the strip to bend and trip the trip mechanism, which breaks the contact and interrupts the Trip Component: This component is a solenoid that generates a magnetic field created when an electric current passes through its coil. When the flow exceeds predetermined levels, the resulting magnetic force is sufficient to move an armature, held back by a spring, to trip the trip mechanism. This locks the Circuit contacts in the open Breaker Types: Circuit breakers are available in three types.

10 Systems designers will choose among inverse time trip, adjustable trip or instantaneous trip Circuit breakers, depending on the Protection Inverse Time Trips: These breakers trip faster as current increases. This provides Overload Protection but also allows equipment and conductors to carry excessive loads Adjustable Trips: These breakers are used when the operation of several Protection Devices in a system must be coordinated. Designers place the lowest rated trips nearest to the Devices being protected so that a fault in one area is isolated but allows current elsewhere in the system to continue to Instantaneous Trips: These use only the magnetic element of the trip and provide no Overload Protection .